1. Field of the Invention
The present invention relates to a rechargeable battery device provided with a function that can accurately determine the operation life of a rechargeable battery such as a nickel hydride metal (Ni-MH) battery which is repeatedly recharged and used as a power source of an electronic device.
2. Description of the Prior Art
In recent years, repeatedly rechargeable batteries have come into widespread use as power sources of electronic devices. However, it cannot be denied that the battery performance of the rechargeable battery deteriorates after repeated charging and discharging or due to secular changes or other reasons. In applications where the rechargeable battery is used as a backup power source of utility power, as seen in an interruptible power supply for a server of a network system, it is important to determine accurately the battery performance and by extension the battery life.
One conventional method of evaluating the performance of a rechargeable battery (degree of cycle deterioration) disclosed in Japanese Unexamined Patent Publication No. Hei 8-228440 uses a discharging load connected via a switch in parallel with the rechargeable battery and determines the battery life by obtaining the internal resistance of the rechargeable battery from a terminal voltage measured while switched off (open) and a terminal voltage measured while switched on (close). This method requires a switch and a load for determining the operation life and requires the discharging of the rechargeable battery, disadvantageously.
Another method disclosed in Japanese Unexamined Patent Publication No. Hei 11-329512 judges the degree of performance deterioration by measuring the constant-current charging time required to bring the rechargeable battery into full charge and by comparing the measured time with the charging time measured when the battery was new. Actually, it is difficult to maintain the conditions for charging the rechargeable battery constant, and the charging time may depend on the temperature of the battery, for instance.
The inventors of the present invention suggested a method of determining the operation life of a rechargeable battery on the basis of the internal resistance of the battery in the full-charge condition (Japanese Patent Application No. 2000-196566). When the rechargeable battery is fully charged, the oxygen gas generated in the battery is absorbed by the negative-pole member, causing a sudden increase in the battery temperature. As the battery temperature increases, the internal resistance of the rechargeable battery varies. Accordingly, to determine the battery life accurately from the internal resistance of the rechargeable battery, as described above, the internal resistance must be compensated for changes in battery temperature, for instance.
An object of the present invention is to provide a rechargeable battery device of a simplified structure which can solve the problems described above and can accurately determine the operation life of the rechargeable battery at stable battery temperatures, without useless consumption of the charged power of the rechargeable battery.
An embodiment of the rechargeable battery device configured to attain the object of the present invention described above comprises battery voltage detection means which detects the terminal voltage of the rechargeable battery comprising a nickel hydride metal battery, for instance, and switching means which switches on and off the charging current continuously or intermittently during the charging of the rechargeable battery. The rechargeable battery device particularly comprises
a memory which stores the battery voltage Von of the rechargeable battery detected by the battery voltage detection means immediately before the charging current is switched off by the switching means,
internal resistance determination means which obtains a value corresponding to the internal resistance R of the rechargeable battery, from the open battery voltage Voff of the rechargeable battery detected by the battery voltage detection means when the charging current is switched off by the switching means, and the battery voltage Von stored in the memory, and
battery life determination means which determines the operation life of the rechargeable battery, in accordance with the value Z(int) corresponding to the early-stage internal resistance R of the rechargeable battery obtained by the internal resistance determination means and the value Z(now) corresponding the latest internal resistance R of the rechargeable battery obtained by the internal resistance determination means.
The internal resistance determination means preferably comprises timer means, and is configured to determine the value corresponding to the internal resistance R of the rechargeable battery, utilizing the open battery voltage Voff of the rechargeable battery obtained by the battery voltage detection means after a predetermined period has elapsed since the switching means switched off the charging current of the rechargeable battery.
Another embodiment of the rechargeable battery device of the present invention comprises battery voltage detection means which determines the terminal voltage of the rechargeable battery, current detection means which detects the charging current to the rechargeable battery, and switching means which switches on and off the charging current of the rechargeable battery continuously or intermittently, and particularly comprises
a memory which stores the battery voltage Von of the rechargeable battery detected by the battery voltage detection means immediately before the charging current is switched off by the switching means, and the charging current Ion detected by the current detection means at the same timing,
internal resistance determination means which obtains a value corresponding to the internal resistance R of the rechargeable battery, from the open battery voltage Voff of the rechargeable battery detected by the battery voltage detection means when the charging current is switched off by the switching means, the charging current Ioff detected by the current detection means at the same timing, and the battery voltage Von and charging current Ion stored in the memory, and
battery life determination means which determines the operation life of the rechargeable battery in accordance with the value Z(int) corresponding to the early-stage internal resistance R of the rechargeable battery obtained by the internal resistance determination means and the value Z(now) corresponding to the latest internal resistance R of the rechargeable battery obtained by the internal resistance determination means.
The internal resistance determination means of this embodiment of the rechargeable battery device preferably comprises timer means, and is configured to determine the value corresponding to the internal resistance R of the rechargeable battery, utilizing the open battery voltage Voff of the rechargeable battery obtained by the battery voltage detection means after a predetermined period has elapsed since the charging current of the rechargeable battery was switched off by the switching means, and the charging current Ioff obtained by the current detection means.
It is preferable that means for prohibiting the calculation process of the internal resistance of the rechargeable battery by the internal resistance determination means when the current detection means detects a discharging current of the rechargeable battery be provided to prevent a wrong determination of the battery life.
If the rechargeable battery comprises a battery block in which multiple rechargeable battery cells are connected in series, the rechargeable battery device of the present invention particularly comprises
voltage detection means which detects the terminal voltage of each group of a single or multiple rechargeable battery cells forming the battery block,
fluctuation determination means which determines the fluctuation in the terminal voltages of the groups,
unbalance detection means which generates an unbalance signal when the fluctuation in the group terminal voltages during or after the charging of the rechargeable battery exceeds a predetermined level, and
battery life determination means which determines the operation life of the battery block, on the basis of the unbalance signal generated by the unbalance detection means.
More specifically, the rechargeable battery device of the present invention has been achieved by paying attention to the fact that the terminal voltages of the rechargeable battery cells vary if the performance of one of multiple rechargeable battery cells connected in series to form the battery block. The terminal voltages of individual groups of a single or multiple rechargeable battery cells are separately detected, and the operation life of the battery block is determined on the basis of the fluctuation of the terminal voltages during or after the charging of the battery block.
The unbalance detection means is preferably configured to determine the fluctuation of the terminal voltages of the group after the lapse of a predetermined period since the beginning of the charging of the rechargeable battery, after the charging of a predetermined amount, after the attainment of a predetermined charging voltage, or after the lapse of a predetermined period since the end of charging.
It is noted that the terminal voltages of the rechargeable battery cells are likely to vary in an early stage of charging even if the performance of the rechargeable battery cells is uniform. Accordingly, the fluctuation of the terminal voltages of the multiple rechargeable battery cells is determined in a period other than the early stage of charging.
In a preferred embodiment of the present invention, the unbalance detection means is configured to determine the fluctuation of the terminal voltages of the groups just once at a predetermined timing during or after charging, each time the battery block is charged. The life determination means is configured to determine the operation life of the battery block, on the basis of the number of times Nt or the number of times in a row Ns the unbalance detection means generates the unbalance signal.
The unbalance detection means may also be configured to determine the fluctuation of the terminal voltages of the group in multiple levels and to generate an unbalance signal according to the determined level. In this case, it is preferable that the life determination means is configured to determine the degree of the operation life of the battery block in accordance with the type of the unbalance signal.